1. Field of the Invention
The present invention relates to a radio frequency (RF) power supply which is optimally designed for both lighting and continuous operation of a CO.sub.2 waveguide laser.
2. Description of the Related Art
CO.sub.2 waveguide lasers are reliable, low cost, fieldable and versatile devices with applications ranging from surgical to military and industrial. A description of these lasers is found in an article entitled "The Coming Age of Waveguide Lasers", by K. Laakmann et al, SPIE vol. 247 Advances in Laser Engineering and Applications (1980), pp. 74-78. These lasers have various configurations, including transverse or longitudinal operating modes, and DC or RF excitation.
A problem with RF excited or driven CO.sub.2 waveguide lasers is that they will not light if the RF excitation is initially applied at the frequency and power level which are optimal for continuous laser operation. For an exemplary laser which operates continuously with maximum efficiency at a frequency of 150 MHz, the RF excitation required to initially light the laser will be on the order of 1 to 5 MHz higher than 150 MHz.
The impedance of the laser is determined by the conditions of the CO.sub.2 gas and the physical dimensions of the electrodes. The gas acts like a dielectric material which forms a distributed capacitor in combination with the electrodes. The dielectric constant of the gas changes from the unlit state to the lit state. If the RF impedance matching between the RF drive power supply and the laser is optimized for the lit state, then in the unlit state the frequency must be changed in order to get enough power into the laser for lighting.
With enough RF drive power, the laser can be started at the optimal frequency for continuous operation. However, this requires a higher power RF supply, which is more complicated, larger and more expensive. Due to the high reflected power during initial lighting of the laser, the RF amplifiers within the RF power supply must be protected. This adds additional complication and cost to the RF power supply.
Various expedients have been utilized for initially applying the RF drive power at an increased frequency for lighting, and then switching the RF drive power to the optimal continuous operating frequency after the laser is lit. A prior art circuit arrangement for providing this function includes two voltage-controlled oscillators (VCOs) operating in an open-loop configuration which generate different RF frequencies at their outputs, and a RF switch for selectively connecting the oscillators to the laser at the proper times.
The first VCO generates a drive signal which initially has a frequency higher than the lighting frequency, and sweeps downwardly past the optimal continuous operating frequency. The second VCO generates a drive signal at the optimal continuous operating frequency. The RF switch connects the first VCO to the laser first for lighting, and then switches out the first VCO and switches in the second VCO after the laser has been lit.
Although it is possible to light and operate a CO.sub.2 waveguide laser using such a frequency switching arrangement, the timing in switching between the two VCOs is critical. If there is a period of time in which the first VCO has been switched out and the second VCO has not yet been switched in (neither VCO is connected to the laser), the laser may go out. Alternatively, if the second VCO is switched in before the first VCO is switched out (both VCOs are connected to the laser), the laser may follow the frequency of the first VCO down below the optimal continuous operating frequency and go out.
VCOs operating open-loop are subject to significant output frequency variation resulting from temperature drift. The frequency versus output voltage of a typical VCO can deviate by 10 MHz to 20 MHz from one device to another and/or due to temperature effects. This deviation is excessive since the difference between the lighting and continuous operating frequencies of the laser is only 1 MHz to 5 MHz. If the frequency does not sweep high enough, the laser will not light. If the frequency sweeps too low, the laser may go out.
In addition, these circuits are not amenable to automated mass production since manual adjustment or "tweaking" is required to tune the VCOs to the correct frequencies.